Conclusions

The results of this study clearly verify the action of the centrifugal acceleration process: (1) The parallel velocity components of H+, He+, He++, and O+, as measured by the TIMAS instrument, all increased by the same amount (about 75 km/s). This result is a necessary consequence of centrifugal acceleration because it is not dependent on the ion charge or mass. Moreover, this velocity jump is consistent with the following ion-tracing results. (2) The backward tracing to the ionosphere of the O+ ions measured before and after the compression reveals that the ions observed at Polar acquired their energies predominantly through the centrifugal acceleration process, not only during the compression, but all along their trajectories. Of course, this process does not account for the initial energy required for the ions to escape from the ionosphere. These tracings were highly constrained: (i) the O+ parallel and perpendicular drift velocities and the local magnetic field were accurately measured; (ii) the ion perpendicular drifts were used to infer the drift along the ion paths, assuming the field lines to be equipotentials and accounting for the higher convection rate at lower field intensities during the compression; and (iii) measurements of the IMF components and the solar-wind pressure were available for inputs to Tsyganenko's magnetic field model. (3) The O+ ions observed both at the beginning and at the end of the compression were traced back to the nominal region of cusp field lines, where their energies and fluxes were typical of quiet-time measurements. This result is consistent with the presence of both ionospheric and magnetosheath ions at Polar during this event. It is also consistent with (2).